The real question is "Why do you want a million numbers in your excel file?" Excel probably can cope, but as long as you are handling the file now, why not do some work on the information before you write it back out? You can use (g)awk to do really quite interesting things: Average of all the numbers above, or below some cut off point, as an example; or a count of numbers in each of various ranges; or sort the numbers (that might be somewhat troublesome for that many numbers). If you are willing to use some other language (Perl, Python, Mathmateca, Maple, C or C++, ...) to handle it, you can (more easily) do even more fascinating things without ever invoking excel.